1. Field of the Invention
Embodiments of the invention generally relate to pressure sensors, and more specifically, to pressure sensors suitable for use in harsh environments.
2. Background of the Related Art
Sensors for the measurement of various physical parameters such as pressure and temperature often rely on the transmission of strain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a sensing element. In a pressure sensor, the sensing element may be bonded to the elastic structure with a suitable adhesive.
It is also known that the attachment of the sensing element to the elastic structure can be a large source of error if the attachment is not highly stable. In the case of sensors that measure static or very slowly changing parameters, the long term stability of the attachment to the structure is extremely important. A major source of such long term sensor instability is a phenomenon known as “creep”, i.e., change in strain on the sensing element with no change in applied load on the elastic structure, which results in a DC shift or drift error in the sensor signal. It is further known that most attachments transmit a base strain to the sensor through the attachment structure and that a true zero base strain sensitivity is difficult if not impossible to achieve.
One example of a fiber optic based sensor is that described in U.S. patent application Ser. No. 9/205,944, entitled “Tube-Encased Fiber Grating Pressure Sensor”, to T. J. Bailey et al., which is incorporated herein by reference in its entirety. Bailey et al. describes an optical fiber based sensor encased within a tube and discloses certain embodiments wherein the sensor is suspended within a fluid. Some examples of such fiber optic sensors include sensors and tubes that are comprised of glass. A disadvantage of such sensors is that the glass is fragile, being brittle and sensitive to cracking. Thus the use of such a sensor in a harsh environment, e.g. where the sensor would be subject to significant levels of shock and vibration, presents a serious threat of damage to the fragile sensor. In certain environments such sensors are subject to shock levels in excess of 100 times the force of gravity (g) and vibration levels of 5 g RMS at frequencies typically ranging from about 10 Hz to about 200 Hz.
However, as discussed hereinbefore, sensor performance is closely tied to attachment techniques and to packaging of the sensor element as well. It is important to package such sensor elements to protect the fragile elements and not impede performance of the sensor in a manner that is reliable and inexpensive to manufacture.
An improved pressure sensor is described in U.S. Pat. No. 6,439,055, issued Aug. 27, 2002, which is hereby incorporated by reference. The pressure sensor generally includes a fiber optic sensing element suspended within a fluid-filled housing. The fluid within the housing maintains the sensing element in a near-zero base strain condition and further protects the sensing element from shock and vibration. Fluid within the housing is coupled by a pressure transmitting device to a fluid surrounding the housing. The pressure transmitting device allows the fiber optic sensing element to provide a metric indicative of the pressure of the fluid surrounding the sensor while maintaining the fluid within the housing in a void free condition. This sensor has demonstrated reliable service in harsh conditions, such as within the well bore of oil and gas wells.
Although this sensor has proven to be a robust and reliable sensing instrument, the overall length of the sensor requires a correspondingly long mandrel which supports the sensor in the well bore of an oil well. As the mandrel has a very high cost-per-unit length relative to a conventional well bore tube section, the length of the sensor (about 28 inches) requires a relatively high cost mandrel. Moreover, as the sensor described in U.S. Pat. No. 6,439,055 is spliced to an optical fiber for communication with surface instruments, the pre-assembly and installation cost of this sensor are undesirably high.
Therefore, there is a need for an improved pressure sensor suitable for use in harsh environments.